The present invention relates to a method and an apparatus for cutting structural steels or mild steels by a laser beam, using at least one lens or at least one bifocal mirror, in order to focus the laser beam at at least two focusing points separate from one another and lying on the same axis, and nitrogen or a nitrogen/oxygen mixture as assist gas for the laser beam.
Structural steels or mild steels encompass alloy or non-alloy steels, whether these are ferritic or austenitic, especially so-called low-alloy steels containing less than about 5% of alloying elements and those so-called high-alloy steels containing more than 5% of alloying elements.
At the present time alloy or non-alloy steels are currently cut by laser beam using oxygen as assist gas, also called cutting gas.
This is because the use of oxygen makes it possible, because of the chemical reaction of iron in the presence of oxygen, to obtain high cutting rates combined with reduced cutting gas flow rates, which means that low-cost cut workpieces are obtained.
However, the use of oxygen as assist gas for a laser beam poses a major problem, namely the problem of oxidation of the cut faces.
This is because it is known that, when cutting alloy or non-alloy steels with a laser beam, the oxygen generates an oxide film on the cut face which prevents the adhesion to it of protective elements, such as paint for example.
In order to obtain better adhesion of such protective elements, a subsequent brushing operation is then required in order to remove this oxide layer.
However, such a post-treatment operation on the cut face or faces considerably increases the cost of the workpiece, this being the more so the more complex the profile of the latter.
Consequently, in order to avoid or minimize this oxidation problem and although oxygen is the most suitable gas for cutting by a laser beam in terms of performance, it has already been proposed to cut steels using nitrogen or a nitrogen/oxygen mixture.
The fact of using nitrogen or a nitrogen/oxygen mixture does indeed make it possible to reduce the oxygen content in the assist gas and therefore the presence of oxides on the cut faces of the workpiece thus cut.
However, with nitrogen or a nitrogen/oxygen mixture, the cutting rates are considerably limited and the gas consumption is increased compared with oxygen.
The use of nitrogen or a nitrogen/oxygen mixture instead of oxygen is therefore not ideal as it results in a reduction in the performance of the laser cutting method.
Consequently, the object of the present invention is to improve the methods of cutting alloy or non-alloy steels by a laser beam, that is to say to provide a method of laser cutting a steel workpiece resulting in little, or indeed almost zero, oxidation of the cut faces by increasing the cutting performance by about 40% compared with a laser cutting method using nitrogen or a nitrogen/oxygen mixture.
The present invention therefore relates to a method of cutting a workpiece made of alloy or non-alloy steel by the use of at least one transparent or reflecting optical means for focusing at least one laser beam and of at least one assist gas for said laser beam, in which the optical means is of the multifocus type and the assist gas is nitrogen or a nitrogen/oxygen mixture containing at least 90% nitrogen.
In the case of the present invention, the expression xe2x80x9coptical means of the multifocus typexe2x80x9d is understood to mean that the optical means, for example a lens, makes it possible to focus the laser beam at several focusing points separated from one another, usually a first and a second separate focusing point, which points generally lie on an axis approximately coaxial with the axis of the nozzle of the laser device, that is to say of the laser head via which the laser beam or beams emanate.
Depending on the case, the method of the invention may comprise one or more of the following characteristics:
the optical means is chosen from lenses, mirrors and combinations thereof, preferably a lens, for example a bifocal lens, that is to say one having two separate focusing points;
the assist gas is nitrogen containing less than 150 ppm by volume of oxygen as impurities, preferably less than 100 ppm by volume;
the assist gas is a nitrogen/oxygen mixture having an oxygen content greater than 0% by volume and less than 8% by volume, preferably between 150 ppm by volume and 5% by volume;
the optical means is arranged so as to obtain at least one first focusing point, being positioned near the upper surface of the workpiece to be cut, preferably so as to coincide with said upper surface, or in the thickness of the workpiece to be cut in a region close to said upper surface, and at least one second focusing point, being positioned near the lower surface of the workpiece to be cut and in the thickness of the latter, or beyond the latter;
the thickness of the workpiece to be cut is between 1.5 mm and 5 mm and, for these thicknesses, a laser source of 1 800 watts in power is then used, for example;
the workpiece to be cut is chosen from plates, sheets and tubes;
the nitrogen/oxygen mixture is obtained directly on the site of use from atmospheric air treated by a membrane system.